SunDose
Sun Exposure Timer
Personalized Sun Exposure Timer
A stopwatch does not know your skin. SunDose builds each session from your skin type, the live UV index and the SPF you actually applied — then keeps time for you.
The short answer
A sun exposure timer tracks how long you spend in the sun and ends the session at a planned point rather than by feel. SunDose calculates that length from three inputs — Fitzpatrick skin type, the current UV index, and the SPF you applied — and prompts you to reapply sunscreen while the session is still running.
The three inputs
Your skin type is the input that moves the number most. The Fitzpatrick scale runs I to VI; type II burns easily and tans minimally, while higher types tolerate more exposure before burning. A generic timer treats every user identically, which is the one thing skin does not do.
The live UV index supplies the conditions. Because UV peaks sharply around solar noon and falls away either side, the same 20 minutes carries very different exposure at 12:00 and 17:00.
The SPF you applied completes the picture — and how much you applied matters as much as the number on the bottle.
Why the SPF number alone misleads
Sunscreen is lab-tested at 2 mg of product per square centimetre of skin, roughly a shot glass for a whole adult body. In practice the Skin Cancer Foundation notes people apply only 25–50% of that amount. SPF 50 spread thinly does not deliver SPF 50.
That is why SunDose asks what you applied rather than assuming, and why the reapply prompt is tied to the running session instead of a fixed alarm — it arrives while you are still outside.
What a session looks like
You set your profile once, then each session runs the same short loop: check the index, confirm your SPF, start the timer, and let the Live Activity keep time from your Lock Screen.
- Durations from 1 to 180 minutes, with the suggested length calculated from your profile.
- Reapply prompts on the session clock, not a wall clock.
- Haptic and notification alerts to flip or reapply.
- Session logged automatically for your history.
Sensitivities shorten the session
Retinoids, exfoliating acids, isotretinoin, tetracycline antibiotics such as doxycycline and several other medications are documented photosensitisers — they make skin react to UV faster than it otherwise would.
Flag them once in your profile and SunDose shortens its suggested session accordingly. If a label mentions sun sensitivity and you are unsure what it means for you, ask a pharmacist — the app gives you a reminder, not a diagnosis.
Frequently asked questions
- How long can I stay in the sun?
- There is no single safe number. It depends on your skin type, the UV index, the SPF you applied and any photosensitising medication. SunDose calculates a session length from your own profile rather than applying a generic figure to everyone.
- How often should I reapply sunscreen?
- The American Academy of Dermatology recommends reapplying at least every two hours, and more often after swimming or sweating. SunDose ties that prompt to the running session so it arrives during exposure rather than at a fixed time of day.
- What is a Fitzpatrick skin type?
- A six-point classification of how skin responds to sun, from type I (always burns, never tans) to type VI. Type II burns easily and tans minimally. Types I and II together make up roughly 35% of the US population.
- Does the timer keep running if I lock my phone?
- Yes. The session appears as a Lock Screen Live Activity, so you can see the countdown and pause or stop it without unlocking the phone or opening the app.
Read more
Sources
- StatPearls, NCBI BookshelfLaser Fitzpatrick Skin Type Recommendations
- The Skin Cancer FoundationHow Much Sunscreen Is Enough?
- American Academy of DermatologyHow to apply sunscreen
- US Food and Drug AdministrationThe Sun and Your Medicine